Elucidating regulatory mechanisms for the expression of an Arabidopsis abscisic acid 8'-hydroxylase gene in response to environmental signals

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Plant hormones are low abundant signal molecules, which significantly impact a variety of processes in plant growth and development. Functional genomics research has revealed that key genetic loci selected by classical breeding for yield and stress responses encode a series of hormone metabolism and signaling genes. The metabolism of plant hormones is regulated by a number of environmental factors and play an important role in tuning plant growth and its response to changing environment. The proposed research program aims to understand the molecular mechanism of how seeds respond to environmental changes: water and after-ripening. The phytohormone abscisic acid (ABA) regulates numerous developmental and physiological processes including seed dormancy and germination. Dry seeds accumulate ABA at a high level, and the ABA levels decrease during seed imbibition (water uptake), which is a critical checkpoint for seed germination. The decrease in seed ABA levels during seed imbibition is regulated by CYP707A2, a gene encoding a P450 monooxygenase responsible for ABA catabolism. The expression of CYP707A2 is regulated by multiple environmental signals, such as nitrate, water, after-ripening, light and temperature. The Nambara lab has studied how the expression of CYP707A2 in seeds is regulated by these environmental signals using Arabidopsis as a model system. Genetic and expression analyses have suggested that transcriptional regulation of CYP707A2 is a hub of environmental signaling and determines whether seeds germinate or not. Seed imbibition is the first step in the seed germination processes. My research group has demonstrated that, during the early stage of seed imbibition, both dormant and non-dormant seeds require de novo transcription and exhibit similar transcriptiome profiles, suggesting that seed imbibition involves active mechanisms in response to water. While the expression of CYP707A2 is up-regulated during seed imbibition, seed imbibition under restricted water uptake reduces the expression of CYP707A2. After-ripening is a period of dry seed storage where seed performance and germination are affected. My research group previously demonstrated that after-ripening enhances the expression of CYP707A2, and mutants defective in CYP707A2 require a prolonged period of after-ripening for germination. However, the molecular mechanisms underlying how these environmental signals affect the expression of CYP707A2 remains largely unclear. We will focus on transcriptional regulation of CYP707A2 in response to water and after-ripening. Moreover, this program will investigate how these environmental factors (i.e. water and after-ripening) affect seed responsiveness to nitrate. Stable seed germination and subsequent seedling establishment under changing environments is a key target for plant biotechnology. Our research will help to gain a novel solution to select elite crop varieties for stable and sustainable agricultural practices.